In a nuclear reactor such as a boiling water reactor (BWR), a plurality of control blades or rods are selectively inserted into and withdrawn from a reactor core containing nuclear fuel bundles therein. The fuel bundles are disposed in flow or fuel channels which are spaced apart from each other to define passages through which the control blades may be translated upwardly or downwardly. Exemplary control blades have cruciform cross sections which are disposed in complementary shaped passages between adjacent fuel bundles. As the control blades are translated upwardly and downwardly, they intermittently slide against the fuel channels.
In order to reduce abrasion between the sliding control blades and the fuel channels, the control blades typically include pin-mounted rollers which provide the sole points of contact between the control blades and the fuel channels as the control blades are translated. In order to reduce wear of the rollers and pins themselves, they are typically formed from conventional abrasion resistant materials containing cobalt. However, it has been determined through operation of the reactors that the neutron flux within the reactor core irradiates the rollers and pins causing them to become highly radioactive and brittle. As the rollers and pins wear and corrode during operation, the particles released thereby remain radioactive and are circulated along with the reactor water channeled through the core. These radioactive particles increase the radiation levels of plant equipment through which the water is circulated.
Accordingly, the cobalt content of rollers and pins is being reduced or eliminated in newer reactors to reduce or eliminate the increased radiation due to the radioactive cobalt particles. However, cobalt-containing rollers and pins are presently in service in nuclear reactors, and the replacement of the entire control rod including the cobalt containing rollers and pins joined thereto would appear to be impractical and costly.
Accordingly, removal of the irradiated rollers and pins and replacement thereof is known in the art. For example, electro-discharge machining (EDM) may be used to sever the welded pins from the control blade in order to remove the rollers. Replacement rollers may then be installed with the replacement pin being rewelded to the control blade. However, since servicing of the control blades must necessarily be done underwater, the EDM and welding processes are, therefore, additionally more difficult to accomplish.
In another example, the rollers and pins may be removed and replaced instead with a spacer or guide pad for reducing the difficulty of the servicing operation. However, the spacer pad must be suitably secured to the control blade for extended usage in the reactor. One known guide pad is a one-piece member screwed into the control blade and welded thereto to prevent its removal during operation. However, the welding increases the difficulty of this servicing method and results in heat affected zones in the control blade.
Another example uses a two-piece guide pad which are joined together through the original roller aperture and swaged or cold rolled together to prevent their disassembly during operation. However, cold working of the spacer material also adds to the difficulty of this servicing method and introduces stresses in the material which may lead to failure in operation.
It is also known to remove old rollers and pins without using EDM, but using a more simple process in which a punch shears away the old roller and pin from the control blade. In this example, a single punch is supported by a frame which rests on top of the control blade, with a pair of spaced apart manual clamps clamping the frame to the control rod to position the punch. The punch is generally aligned with one of the rollers so that upon actuation of the hydraulic cylinder which translates the punch, the roller is severed from the control blade. However, the punch and frame are an unbalanced assembly which makes alignment of the punch and roller difficult underwater, and the manual clamps also diminish the ability to obtain accurate alignment. Accordingly, a portion of the control blade itself may also be severed away. This is not a concern where the irradiated rollers are being severed from the control blade for disposing of the control blade after its useful life. Damaging the control blade is a significant concern if it has a remaining useful life and the rollers are to be replaced with spacer pads, for example, for continued operation.